This invention relates to a system and a method of securing the end shields of a dynamoelectric machine (e.g., fractional horsepower electric motor) to the stator assembly of the motor with the rotor shaft journalled in bearings carried by the end shields and with the rotor properly positioned (i.e., centered) within the bore of the stator assembly so as to provide a uniform air gap therebetween.
Typically, a fractional horsepower motor includes a stator assembly having a stack of laminations (preferably made of sheets of suitable steel or other ferro-magnetic material) punched to have a central bore and a plurality of winding receiving slots extending generally radially outwardly from the bore. The laminations are stacked to form a core and are securely fastened together. Welding or epoxy adhesive work well as fastening methods, for example. A rotor assembly including a rotor, typically of a squirrel cage design, and an axial shaft extending from both ends thereof is provided. The rotor is intended to be received within the bore of the stator core so as to be centered therewithin and to have a substantially uniform air gap between the rotor and portions of the stator core forming the bore. These motors typically include a pair of end shields adapted to be secured to the ends of the stator assembly. The end shields each include a bearing (e.g., a suitable anti-friction bearing) in which one end of the rotor shaft is journalled so that the bearings rotatably support the motor within the stator assembly.
Heretofore, a variety of means and procedures have been used to locate (i.e., center) the rotor within the bore of the stator assembly and to secure the rotor in this centered position. Typically, during manufacture of such an electric motor, a rotor is inserted into the bore of its respective stator assembly and temporary shims are fitted so as to accurately center the motor with respect to the bore. The bearings and the end shield are then fitted on the rotor shaft and the end shields are fitted on the stator assembly. With the end shields and the rotor in proper position, the end shields are then secured to the stator and the temporary shims are removed.
Certain of the various means heretofore used to secure the end shields to the stator assembly will be now discussed. In many motors, so-called through-bolts extend through openings in the stator core and through openings in both of the end shields. Typically, the shanks of the through-bolts are somewhat smaller than the diameter of the openings in the stator core and the openings in the end shield so as to permit the end shields to be shifted relative to the stator core thereby to enable the rotor to be properly centered within the bore of the stator assembly and so as to provide the desired uniform air gap therearound. Through bolts are then tightened to a predetermined torque level while the rotor is shimmed in its desired centered location. The degree to which the through-bolts are tightened (i.e., a predetermined torque level) preloads the bolts so as to apply a clamping force between the end shields and the stator assembly. As the end shields are drawn into firm engagement with the ends of the stator assembly the friction between the end shields and the stator assembly holds the end shields in place.
Certain problems, however, have been noted with this above-noted prior art system of securing the end shields in position on the stator assembly. First, the cost of the through bolt fasteners is relatively high. Second, only the friction of the end shields on the stator assembly holds the rotor centered within the bore of the stator. If the rotor shaft or end shield shifts either during shipping or during use of the motor, it is possible for the rotor to move from its centered position thus upsetting the uniform air gap between the rotor and bore of the stator. In some instances, if the air gap becomes too small or if the rotor physically touches or "strikes" the stator, the motor fails. Since the end shields of these prior art motors are dependent entirely on the frictional force between the end shields and the stator core to hold them firmly in place, the impact force which the end shields will withstand and yet hold the rotor centered is dependent on the preload of the through-bolts. Prior motors utilizing through-bolt construction have experienced problems with the preload of the through-bolts relaxing over time. This is due in part to the fact that the stator assembly is typically dipped in an insulating varnish solution and dried. The varnish, of course, coats the outer end faces of the stator assembly and is present between the laminations of the stator assembly. When the through bolts are drawn tight, the varnish between the end shields and the stator core and the varnish between the laminations tends to seep out under compressive load, but still may be present after varnish set. In use, motor operating temperatures can soften the varnish so that it again becomes flowable and "creeps" under the compressive load. This creeping of the varnish causes the preload in the through-bolts to relax, thus lessening the frictional force available to hold the end shield in place on the stator core.
A second method or system of securing the end shields to the stator assembly involves bonding the end shields to the stator assembly once the rotor and end shields have been properly positioned relative to the stator and once the rotor is centered relative to the stator bore. Typically, a suitable epoxy adhesive or the like is used to bond the end shields in place to the stator assembly. While this method of bonding the end shields in place may offer some advantages in manufacture, improperly bonded end shields may easily be separated from the stator assembly if inadvertently struck during shipping or during use of the motor. Also, there is no ready or practical way to field repair or to economically rebuild the motor with bonded in place end shields.
Reference may be made to such U.S. Pat. Nos. as 2,423,750, 3,176,172, 3,320,660, 3,437,853, 3,707,037 and 3,966,278 which disclose various methods and systems of securing end shields in place to the stator assembly of electric motors or the like.